Information and communication technology (ICT) innovation has recently had remarkable growth and development and is shifting toward optical signal communications from electric signal communications in order to accommodate needs of higher communication speed and increasing amount of information with proliferation of the Internet. Many trunk cables, collecting much information from various relay points, are being replaced with optical cables, thus having remarkably improved processing speed. Because communications between the optical cables and users' terminals will be reviewed from this time forward, the demand for more inexpensive and comfortable environmental arrangement of information and communication technologies is increasingly intensifying.
With the enhancement of optical communication networks, delivery and receipt of much information have been possible to make at higher speeds. Accordingly, development of new applications is on the rise and the amount of information being exchanged through optical communication networks has been further increased. To increase the amount of information to be processed by optical fibers, there has been presently used a technology for increasing quantity of signals per unit time by using high-frequency signals and simultaneously transmitting signals having various wavelengths about different types of information through one optical fiber, which is called a wavelength multiplexing system. For formation of a precise and highly reliable communication network, it is necessary to ensure multidirectional connection to a plurality of paths and use of a plurality of optical fibers is essential from the viewpoint of maintenance.
For formation of an optical communication circuit for transmitting a great many signals through an optical fiber, a Wavelength Division Multiplex (hereinafter referred to as “WDM”) system is essential which divides optical signals subjected to wavelength multiplexing into respective wavelengths, and instead multiplexes optical signals of different types of wavelengths, and further branches or inserts optical signals. Accordingly, the amount of information has been increased and the importance of information to be handled has been more significant. If an optical signal has some fault, it is necessary to promptly grasp where and which optical signal has the fault and to verify optical signal intensity as needed in addition to presence of optical signal connection. Moreover, if a transfer distance is long, the optical signal intensity attenuates, which requires an Erbium Doped Fiber Amplifier, hereinafter referred to as “EDFA” for amplifying the optical signal. EDFA needs to exactly grasp the intensity of an optical signal inputted from the outside for evaluation of an amplification ratio and the intensity of an optical signal outputted to the outside after the amplification. Consequently, to develop an optical communication system with high reliability, such a precise monitoring function is indispensable.
The WDM system has fixed input and output directions of optical signals. Monitoring of the optical signals has required no particular directivity. On the other hand, the EDFA receives pump laser and propagates it in a special fiber for amplification of the optical signals. Accordingly, the amplified optical signals sometimes have their backflows and, to exactly determine the amplification level of the optical signals, a function is essential which detects only the optical signal from input fiber and will not detect the light returned from an output fiber.
A conventional monitoring method for general optical signals uses a technology which branches part of the optical signals with a photocoupler and detects the branched optical signals with a photodiode. Accordingly, fused connection of each component is required, which causes interference with reduction in mounting manpower. Moreover, the photocoupler has a structure which branches optical signals by approaching cores serving as optical signal propagating sections of optical fibers and the length of the approaching section is one of key parameters of a branching amount. This makes it difficult to miniaturize each of products, thus impairing reduction in component sizes. Recently, there are strong needs for downsizing the EDFA apparatus and no reduction of component size leads to restrictions on the miniaturization and high package density of the EDFA apparatus.
Patent Document 1 discloses an example of a bi-directional optical power monitor miniaturized for easy handling. FIG. 6 is a structure of the disclosed monitor. A multi-capillary glass ferrule 53 (equivalent to a pig tail fiber) having two optical fibers 51, 52, respectively (an input optical fiber 51 and an output optical fiber 52, respectively) and GRIN (Gradient Index) lens 54 are made to face each other through an air gap 55 with a predetermined length. On an end surface of the GRIN lens, a filter 56 (equivalent to a tap film) is provided to permit the light passing through the GRIN lens to reflect and penetrate. The light transmitting through the filter passes through an air gap 57 and is converted into an electric signal by a photon detector 58 (corresponding to a photodiode) to measure the intensity of the light inputted into the optical fiber. The multi-capillary glass ferrule 53 and the GRIN lens 54 are retained with glass tubes 60, 61. Because both the two optical fibers 51, 52 permits light inputs and outputs, this apparatus may be called a bi-directional optical power monitor. The GRIN lens is a glass column of which refraction factor varies radially and continuously toward the outer-periphery direction from the center line. The refraction factor becomes larger as it is nearer to the outer periphery, and as the light expands widely to the outer periphery, the traveling direction of the light is biased in the center line direction, so that the penetrating light gathers around the filter center.
The light incident into the air gap 55 from the input optical fiber 51, passing through the GRIN lens 54, reaches the filter 56 on the end surface of the GRIN lens. Most of the light which has reached the filter 56 reflects, passes through the GRIN lens 54 and the air gap 55 and enters the output optical fiber 52 to produce output light. Part of the light which has reached the filter 56 transmits through the filter 56, passes through the air gap 57, enters the photon detector 58 and is converted into an electric signal for output. Such a series of optical paths are indicated by solid-line arrows. On the contrary, when the light is entered from the output optical fiber 52, it has the same passage as the above-described optical path, so that the light can be removed from the input optical fiber 51. The light which has penetrated through the filter 56 passes through the air gap 57, enters the photon detector 58 and is converted into an electric signal for output. Such a series of optical paths are indicated by broken-line arrows.
Non-patent Document 1 discloses an example of an optical power monitor having uni-directionality. FIG. 7 illustrates a structure of the disclosed power monitor. The names of parts use those used in Non-patent Document 1. A two-core ferrule 80 (equivalent to a pig tail fiber) having two of an input optical fiber 81 and an output optical fiber 82 called as a port 1 and a port 2 respectively is butted against a GRIN lens 83. On an end surface of the GRIN lens 83, a dielectric mirror 84 (equivalent to a tap film) is formed to conduct reflection and transmission of light. The center line of the GRIN lens is disposed, shifted from that of the photo-detector 85 (equivalent to a photodiode).
The following is an explanation of an optical flow. The light (input light) incident from the input optical fiber 81 (port 1) passes through GRIN lens 83 and reflects and penetrates using the dielectric mirror 84. The reflected light passes through the GRIN lens and enters the output optical fiber 82 (port 2) to produce output light. The light, which has transmitted through the dielectric mirror, enters a photo-detector 85, is converted into an electric signal and outputted as an electric signal. Such a series of optical paths are indicated by solid-line arrows. Next, the light coming from the output optical fiber 92 (port 2) will be described below. The light coming from the output optical fiber 82 passes through the GRIN lens 83 and is reflected by and is made to transmit through the dielectric mirror 84. The reflected light passes through the GRIN lens again and enters the input optical fiber 81 (port 1) to become the output light. The light, which has transmitted through the dielectric mirror, does not enter the photo-detector 85 but is discharged to the outside because the optical axis (center line) of GRIN lens is shifted from the optical axis (center line) of the photo-detector 85. Accordingly, the intensity of the incoming light from the output optical fiber 82 (port 2) is impossible to measure. Such a series of optical paths are indicated by broken-line arrows. In other words, there is used an optical power monitor which has uni-directionality, that is, the following phenomenon: the intensity of the incoming light from the input optical fiber 81 (port 1) is possible to measure and the intensity of the incoming light from the output optical fiber 82 (port 2) is not possible to measure.
In the directional characteristic of the uni-directional optical power monitor, a ratio of the optical sensitivity A (μA/w) of an photodiode obtained when light is inputted from the input optical fiber to the optical sensitivity B (μA/w) of an photodiode obtained when the same light is inputted from the output optical fiber is expressed as a unit of dB and determined from the directional characteristic=10 log10(A/B). The uni-directional optical power monitor requires a directional characteristic of at least 25 dB.
For the uni-directional optical power monitor illustrated in FIG. 7, a positional relationship between the optical axis of the GRIN lens and the optical axis of the photodiode is described, however, a detailed structure of the optical path between the GRIN lens and the photodiode is not described. The uni-directional optical power monitor requires to position and fix the GRIN lens and the photodiode with a sleeve or the like. If the GRIN lens approaches the photodiode too much, even the light transmitted from any optical fiber is detected, therefore the GRIN lens should be distant from the photodiode by at least a certain distance.
Patent Document 1: U.S. Pat. No. 6,603,906
Non-patent Document 1: Preprint, Lecture No. C-3-51, page 183, FIG. 3, for the Institute of Electronics, Information and Communication Engineers in Japan held on Mar. 28, 2002